Self-clocking five bit record-playback ststem

ABSTRACT

1. A SYSTEM FOR MAGNETICALLY RECORDING AND REPRODUCING DIGITAL DATA COMPRISING: (AN ENCODER FOR CONVERTING A FOUR-BIT BINARY CODE TO A FIVE-BIT BINARY CODE ALLOWING COMBINATION OF TWO SUCCESSIVE BITS ONLY AS LIKE BINARY DIGITS;) A RECORDING MEDIUM; RECORDING MEANS (CONNECTED TO SAID ENCODER) FOR RECORDING DIGITAL DATA IN (SAID) A FIVE-BIT BINARY CODE ON SAID RECORDING MEDIUM, WHEREBY THE POLARITY OF SAID RECORDING MEDIUM IS REVERSED TO RECORD ONLY ONE OF A PAIR OF BINARY DIGITS; AN ENCODER CONNECTED TO SAID RECORDING MEANS FOR CONVERTING A FOUR-BIT BINARY CODE TO SAID FIVE-BIT BINARY CODE ALLOWING COMBINATION OF ONLY TWO SUCCESSIVE BITS AS THE SECOND OF SAID PAIR OF BINARY DIGITS; PLAYBACK MEANS FOR SENSING SAID REVERSALS OF POLARITY OF SAID RECORDING MEDIUM AND GENERATING PULSES IN RESPONSE THERETO; AN OSCILLATOR CONNECTED TO SAID PLAYBACK MEANS EFFECTIVE TO GENERATE A WAVE HAVING A FREQUENCY DETERMINED BY SAID PULSES; AND LOGIC MEANS CONNECTED TO SAID PLAYBACK MEANS AND TO SAID OSCILLATOR FOR RECOVERING SAID DIGITAL DATA.

Fab. 4, 1975 G- E. MILLIGAN SELF-CLOCKIHG FIVE BIT RECORD-PLAYBACK SYSTEM riginal Filed Aug. 17, 1970 2 Shents-Shent 1 c Af.'

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Dfrfrrak United States Patent O 28,330 SELF-CLOCKING FIVV. BIT RECORD-PLAYBACK SYSTEM Gene E. Milligan. Torrance, Calif., assigner to NCR Corporation` Dayton. Ohio Original No. 3,641,525, dated Feb. 8, 1972. Ser. No. 64,355, Aug. 17, 1970. Application for reissue Jan. 30, 1974, Ser. No. 438.027

Int. Cl. Gllb 5/'02 U.S. Cl. S60-48 13 Claims Matter enclosed in heavy brackets appears in the original patent but forms no part of this reissue specification; matter printed in italics indicates the additions made by reissue.

ABSTRACT OF THE DISCLOSURE A magnetic recording and reproducing system wherein a parallel four-bit input code is converted into a five-bit character code and magnctcely recorded. The code has BACKGROUND OF THE INVENTION The present invention relates to a recording and reproducing system and more particularly to an improved digital recording and reproducing system for having high etliciency, a low bandwidth, and is self-clocking.

Normally, digital devices are provided with at least one storage device adapted to store a relatively large volume of digital information without modifying the information. Magnetic media such as tape, discs, cards. drums, etc.. are commonly employed in connection with such storage devices. Digital information is recorded on the magnetic medium as either of two magnetic Flux patterns which sequentially occur at discrete points. Normally, at least one of the linx patterns includes a magnetic ux change which may be either complete reversal of polarity or a change from one level of magnetization to a second level.

Because of timing variations between the equipment for recording and that for reproducing the digital information, speed variations of the media, ilutter, etc., a clock pulse is normally employed to read data from the magnetic medium. The clock pulse may be recorded on a separate channel of the magnetic medium, or a continuously running clock pulse generator is synchronized by the pulses produced by the llux changes of the recorded digital information. ln this way the clock pulses have the same timing variations as the recorded digital information.

For reasons of economy and etiiciency, as many digits as can be reliably reproduced are recorded on a unit length of a magnetic medium. As will be apparent, it becomes more ditiicult to reliably reproduce digits as the digits are recorded closer together because of the electrical and mechanical limitations of the recording and reproducing system. One such limitation on storage density is that, as the storage density is increased. the number of flux patterns per unit length of magnetic medium is correspondingly increased. and hence, the number of Eux changes per unit length is increased. A reproducing head has an output which is proportional to the rate of change of the ux of the magnetic medium. Therefore, each Flux reversal is rcproduced as a pulse by the reproducing head. As the storage density is increased. the distance between reproduced pulses is decreased. As a result, wavelength is reduced and the frequency of the reproduced signal is correspondingly increased. It is apparent that the bandwidth of reproducing ampliers must be correspondingly increased.

Since the wavelength resolution of the reproducing system is limited, there is a limit to the number of flux changes per length of magnetic medium that can be reliably reproduced. Accordingly the density of ux changes is limited for a given recording and reproducing system. For maximum data storage density. the minimum possible number of ux changes is employed to represent the digital information.

Another limitation on the storage density is the increased possibility of not reproducing ux changes as the number of flux changes per length of medium is increased. This phenomenon is known as tape dropout error. Dropout error arisesl for example, when the wavelength of the recorded pulses approaches the size of the airgap of the reproducing head. As the density of the tlux changes is increased, the changes representing the digital information is increased.

In digital recording systems heretofore known to the art, digital information has been recorded on the magnetic medium by employing either the return to zero" method. the nonreturn to zero method, or the phase shift or Manchester method.

In the return to zero method of recording digital information, one state of magnetization of the magnetic medium is assigned to the digit 1" and the Opposite state is assigned to the digit 0. Ordinarily, the magnetic medium is maintained in one state of magnetization. It is pulsed to the opposite state and back again to the original state to record the occurrence of the digit 1. Hence, it is necessary to record two ilux reversals for each unit digit. As the recorded pulses are packed closer together. adjacent pulses interfere with one another. It is necessary to leave a space between pulses which is large relative to the duration of the pulse. A given reproducing system can reliably read ux reversals which are further apart than a minimum distance. Consequently, the maximum number of digits per unit length of magnetic medium that can be recorded using the return to zero" method, is relatively low.

In the conventional nonreturn to zero method of digital recording, no tixed state of magnetization is assigned to 1 or 0, Instead, the state of magnetization is reversed each time the digit 1" is recorded and is retained unchanged to indicate the recording of the digit 0. It is apparent, therefore, that one flux reversal is required for each occurrence of the digit l and no ux reversals are required for the digit 0. Therefore the number of digits per length that may be recorded by this recording method is large. However. major problems arise as the ux reversals are packed closer together. One of the major problems stems from the limited resolution of the reproducing system. Variations and spacing between flux reversals may cause the reproduced pulses to merge into one another, or to stretch over the 0" areas. where no pulse is to be reproduced. Another e'ect resulting from the limited resolution is that the reproduced signal is large when the spacing between flut reversals is wide and small where the spacing is close. Therefore. the nonreturn to zero method. because of the limited resolution of the reproducing system,

results in diculties in detecting the absence or presence of pulses as the number of digits recorded per unit length of magnetic medium is increased.

Another limitation on the maximum possible ilux reversal density is due to the fact that the ux reversals occur at random depending on the composition of the digital information. As a result, the flux reversals are not sufficiently continuous to be employed to synchronize a clock pulse source. Therefore, a separate clock pulse channel must be recorded on the magnetic tape, the clock pulse being utilized to read the data channels.

In the Manchester or phase-shift method, the digit 1 is recorded as a single cycle of the square wave and the digit "0" is recorded as a single cycle of the square wave shifted 180 from the l square wave. It will be seen that flux reversal in one direction is employed to indicate the digit 1" and a ux reversal in the opposite direction is employed to indicate the digit 0. This method has the advantage that a ux reversal is provided for each digit whether it is a "0" or a 1. Therefore, the ux reversals may be employed to synchronize a clock pulse source. Errors, such as may be caused by tape skew, are eliminated.

However, the Manchester method has the disadvantage that when reading the fiux reversals, it is necessary to sense the direction of ux reversals to determine whether a digit is a 1" or a "0." Therefore, information dropout always causes an error in this method. Another inherent disadvantage is that two fiux reversals are sometimes necessary to record one digit of digital information,

Since there is a limit cn the number of fiux reversals that can be reliably reproduced, the maximum possible storage density is limited. The "Manchester" method is only 50 percent eicient, since a clock transition is recorded for each data transition. Because of the 50 percent eiciency of the Manchestertype recording, during reproduction a tolerance of only plus or minus 25 percent of the duration of a bit cell can be allowed for timing error.

SUMMARY OF THE INVENTION In the present invention a recording and reproducing system is provided approaching the efficiency of nonretum to zero while retaining the self-clocking and low bandwidth properties of Manchester"type recording. An additional bit is added to the four data bits in a four-digit binary code to provide a ve binary digit code. Transitions in the magnetic state are recorded at the center of each bit cell representing zero. On the other hand, there are no transitions of the magnetic state in the bit cell when "ones are recorded. Means are provided for recording digital data whereby the data can be successfully recovered with a timing error of up to plus or minus 50 percent of a bit cell as recorded. Taking into account the 20 percent redundancy resulting from the use of the additional binary digit, a timing error of plus or minus 4l) percent of each of the four binary digit data bit cells is permissible. System bandwidth is minimized, contributing to the minimizing of the timing error.

The five-digit code into which the four binary digit data is converted is arranged so that no more than two binary "ones follow one another. Further implementing this rule. both the first and second binary digits cannot be ones, nor can both the fourth and fifth binary digits be ones." Each zero is recorded with a flux reversal, generating a recording pattern with three possible wavelengths between ux reversals for any combination of characters. It will be apparent, therefore. that reversals of the state of magnetization occur only 621/5 percent as closely as are required with Manchester"type recording.

The present invention comprehends a recording circuit and a reproduction circuit operating in conjunction with a suitable magnetic recording medium such as tape or cards. The recording circuit, after conversion from the conventional four-bit binary decimal code to a five-bit low redundancy code, serializes the code groups with the aid of a clock generator. The serialized five-bit code is applied to a flip-Hop giving a square wave output, which is amplified and applied to a recording head, recording magnetic state transitions in accordance with the tive-bit code on the magnetic medium.

The reproduction means forming part of the present invention includes a reproduction head cooperating with the magnetic medium to convert the recorded transitions into electrical signals` and a reproduction circuit for converting the recorded electrical signals from the reproduction head into the original four bit binary decimal code. The reproduction circuit includes a phase-locked oscillator connected to run in synchronism with the pulses from the reproduction head. Pulses from the reproduction head and from the phase-locked oscillator are applied to a plurality of gate circuits and Hip-Hops to recover the recorded fivebit code. The serial live-bit code data is converted to parallel form. A parallel decoder is provided to convert the fivebit code back to the original four-bit binary coded data supplied to the recording circuit.

It is, therefore, an object of this invention to provide a high-density, self-clocking. magnetic recording system.

Another object of this invention is to provide a magnetic recording system having low bandwidth requirements. together with high bit density.

Another object of this invention is to provide a magnetic recording system having increased timing tolerances.

Another object of the present invention is to provide a magnetic recording system wherein greater tolerances in the recorded signal positions are enabled.

Another object of this invention is to provide a highly efcient. simple. inexpensive digital magnetic recording and playback system.

BRIEF DESCRIPTION OF THE DRAWINGS These and other objects and advantages of the present invention will become apparent by reference to the following description and accompanying drawings wherein:

FIG. l is a block diagram of a digital recording and reproducing system in accordance with the present invention:

FIG. 2 is a code conversion table illustrating the rules for converting a four-digit binary code to the five-bit code employed in connection with the present invention. and,

FIG. 3 illustrates various waveforms occurring in the operation of the system of FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENT Referring now to the drawings, and particularly to FIG. l, information in the four-bit binary decimal inputoutput code illustrated in FIG. 2 is applied to the recording circuit in parallel on the four input lines 11a, 1lb. llc and 11d. An encoder 12 accepts the four-bit input code and converts it to a five-bit illustrated in FIG. 2.

The five-bit code in which the fourbit binary decimal code is converted is a low redundancy code constructed in accordance with three rules: 1. No more than two 1'5" can occur in succession to one another;

2. The first and second bits cannot both be binary ls"; and,

3. The fourth and fifth bits cannot both be binary l`s. There are 17 possible code combinations obeying these rules. Of these, 16 are employed to uniquely specify decimal numerals from 09. and six arbitrary alphabetical characters. illustrated in FIG. 2 as a, b, c, d, e, and f. The 17th combination is employed as an idle or synchronizing character.

The parallel five-bit code from encoder 12 is applied over cable 13 to a serializer 14. The parallel input is cortverted into a. serial output by serializer 14. The serial ve-bit code is applied to AND-gate 15. Clock pulses from a clock generator 16 are also applied to AND-gate l5.

Clock generator 16 is connected to provide timing pulses to encoder 12, to serializer 14, and to AND-gate 15. The pulses applied to serializer 14 by clock geneartor 16 are of the same frequency as those applied to AND-gate 15. but are delayed in phase by a half-cycle, as illustrated by waveforms c and d in FIG. 3. The serialized data pulse train from serializer 14 in the live-code of FIG. 2, is illustrated in FIG. 3e. The pulse train from serializer 14 is applied to AND-gate 1S. together with pulses from recording clock generator 16. Gate 1S transmits pulses from recording clock generator 16 in accordance with the serialized data in the five-bit code from serializer 14.

The cotte pulse train is applied to a flip-flop 17. Flipop 17 alternately sets and resets. as actuated by the data pulses coming from AND-gate l5. The rectangular nave output from dip-flop 17 is illustrated as FIG. 3f.

The resultant data bearing rectangular wave of FIG. 3f is applied to a magnetic recording head 22 through a suitable "write" amplifier 21. Recording head 22 records the square wave of FIG. 3f on a magnetic medium 23.

The playback section of the present invention is illustrated in the lower portion of FIG. l. A reading head 24. positioned in proximity to the moving record bearing magnetic medium 23, senses the changes in the magnetic tield on the medium caused by the recording thereon of the waveform of FIG. 3f.

The resultant reversals of magnetization cause pulses to be induced in playback head 24. which are applied to a peak detector 25. Peak detector 25 senses the pulses from playback head 74 generated by the alternations of magnetization of the square wave in magnetic medium 23. and serves to effectively sharpen the pulses. The pulse output of peak detector 25 is illustrated at FIG. 3g. These pulses are applied to a phase lock oscillator 26. providing a synchronization signal to the phase relationship of the output of phase lock oscillator 26, illustrated as FIG. 3h, is constant in frequency and stable'in phase. The pulse output from peak detector 25 is also applied to AND-gates 27 and 32. The oulp'ut signal from phase lock oscillator 26 is applied directly tiip-fiop 34, and through inverter 3S, to AND-gates 31 and 33. As

will be further discussed hereinbelow` the output of phase set terminal of ip-liop 36. The reset terminal of flipflop 36 is connected to the output of AND-gate 3l. Similarly, the set terminal of ip-op 37 is connected to the output signal from AND-gate 32 and the "reset terminal is connected to the output from AND-gate 33.

One output of flip-flop 36 is connected to an input terminal of AND-gate 41 and is illustrated by FIG. 3h. The

other input terminals of AND-gate 41 are connected to V.

an output of flip-hop 34 and to the output signal from phase lock oscillator 26.

AND-gate 42 is connected to the output of flip-[iop 37. The waveform produced by Hip-flop 37 is illustrated at FIG. 3o. AND-gate 42 is also connected to the output of phase lock oscillator 26, and to the other output of tiiptiop 34. AND-gate 43, as well as AND-gate 44, are also connected to phase lock oscillator 26. In addition, AND- gate 43 is connected to flip-flop 36 and to flip-flop 34.

AND-gate 44 similarly is connected to Hip-Hop 37 and y ip-op 34. The output waveforms from AND-gates 41, 42. 43, and 44 are illustrated by FIGS. 3p, 3q. 3r, and 3s respectively. AND-gates 41 and 42 are connected to one input of ip-flop 45, while AND-gates 43 and 44 are connected to the other input of tiip-op 45. The output signal from ip-tiop 45, as illustrated at FIG. 3t, is representative of the output signal from serializer 14, and is the serial conversion of the data transmitted in the tive-bit code. This signal is then applied to a deserializer 46, wherein the serial tive-bit code is converted to parallel form and applied over lines 47 to decoder 51. Decoder Sl serves to reconvert the five-bit binary code to the four-bit binary-decimal input-output code, which is transmitted in parallel over the output lines 52 to a suitable utilization device such as a digital computer.

Phase-locked oscillator 26 alternately conditions AND- gates 27 and 33 or `32 and 31 through ip-tiop 34. A recorded pulse reproducing a binary "0" sensed by reproducing head 24 will thus set either of ip-ops 36 or 37 alternately. The lack of a pulse representing a binary "l" illator. Thefrequency and t lll G maintains tiip-tiops 36 and 37 in their "reset" condition. and if previously set," restores them to "reset."

Gates 4l, 42, 43 and 44 combine the outputs from ipops 36 and 37, phase-locked oscillator 26 and ip-liop 12 enable combining the binary signals from the alternate ip-ops 36 and 37. Since the duty cycle of the logic elements is halved due to the alternating arrangement. the pulse timing is not critical. and may vary within half the length of time Hip-Hop 34 provides a positive output. Tolerance of pulse position in each cell on the recording tape is such that a pulse appearing substantially any\\ here within the entire cell width is accurately handled. Further assurance of accuracy is provided in that the logic elements not being employed at a given time act as checks on the accuracy of the logic elements actually operating.

What is claimed is:

1. A system for magnetically recording and reproducing digital data comprising:

[an encoder for converting a four-bit binary code to a ve-bit binary code allowing combination of two successive bits only as like binary digits;]

a recording medium;

recording means [connected to said encoder] for rccording digital data in [said] a five-bit binary code on said recording medium, whereby the polarity of said recording medium is reversed to record only one of a pair of binary digits;

an encoder connected to said recording means for converting a four-bit binary Code to .raid five-bit binary code allowing combination of only two sncccssive bits as the second of said pair of binary digits;

playback means for sensing said reversals of polarity of said recording medium and generating pulses in response thereto;

an oscillator connected to said playback means effective to generate a wave having a frequency determined hy said pulses;

and logic means connected to said playback means and to said oscillator for recovering said digital data.

2. A digital recording and reproducing system comprising:

means for supplying binary input data with alphanumeric characters represented by a four-bit code;

first code conversion means effective to convert said four-bit code into a low redundancy tive-bit code comprising combinations of binary digits wherein no more than two [similar selected] of a nrs! one nf a pair of binary digits occur in succession;

magnetic recording means for recording [said selected] the rst one 0f said pair 0f binary digits as the absence of a magnetic state change on a magnetic nzedium and the second one of said pair of binary digits as a magnetic state [reversal] change on [a] snit! magnetic medium;

reproducing means detecting said magnetic state [reversals] changes on said magnetic medium as electrical pulses;

oscillator means having a frequency determined by said electrical pulses;

logic means combining said pulses and the output of said oscillator means to reproduce said tive-bit code; and

second code conversion means effective to convert said tive-bit binary code to said four-bit binary code.

3. In the recording and reproducing system of claim 2. each of said binary digits occupying a cell space on said magnetic medium.

4. In the recording and reproducing system of claim 3. said reproducing means including a peak detector connected to a magnetic playback head.

S. In the recording and reproducing system of claim 4, said oscillator means including a phase-locked oscillator driven by said electrical pulses and generating output pulses having a frequency and phase relationship determined by pulses detected by said reproducing means.

6. In the recording and reproducing system of claim 5, said logic means including:

first and second gate means alternately conditioned by means responsive to said phase-locked oscillator and responsive to said reproducing means; and

rst and second means responsive to said first and second gate means, respectively, for responding to alternate digit cell spaces. 7. ln the recording and reproducing system of claim 6, said rst and second means including:

a rst ip-ilop connected to sain first gate means and a second ip-fiop connected to said second gate means; third gate means responsive to said first and second ipflops and to said phase locked oscillator for recombining said reproduced pulses into said five-bit code. 8. In the recording and reproducing system of claim 7. a deserializer connected to said third gate means for converting said ve-bit code to parallel form, to enable said [decoder] second code conversion means to convert said five-bit code into said four-bit code.

9. A system for magnetically recording and reproducing digital data comprising:

an encoder translating digital data from a four-bit binary code to a five-bit binary code wherein only two successive bits may [both] be [binary ones] a first of a pair of binary digits.' y

circuit means connected to said encoder effective to provide signals successively alternating in polarity upon occurrence of a [preselected one of said binary digits] second of said pair of binary digits in said vebit binary code; recording means connected to said circuit means for recording said signal alternating in polarity in said vebit binary code on said recording medium; t,

reproducing means cooperating with said recording medium for sensing said signals alternating in polarity in said five-bit binary code;

oscillator means connected to said reproducing means for generating a signal synchronized to said signals alternating in polarity; and

logic means in circuit with said reproducing means and said oscillator means for translating said live-bit binary code to recover said data in said four-bit binary code.

10. The method of magnetically recording and repro` ducing digital data on a magnetic recording medium comprising:

providing digital data in a four-bit binary code;

converting said digital data from said four-bit binary code to a five-bit binary code wherein only two successive bits can be [the same] a first binary digit,- serially recording said digital data in said five-bit binary code on a magnetic medium with the state of magnetization [reversed each time a rst digit is recorded,

and] allowed to remain in the state previously recorded each time [a second] said rst digit is recorded, and changed each time a second digit is recorded;

sensing said magnetic medium to detect said [reversals] changes of state of magnetization as pulses;

driving a phase locked oscillator with said pulses to provide a square wave having a frequency and phase relationship determined by said pulses;

combining said square wave with said pulses to recover said digital data in said five-bit binary code; and

converting said five-bit binary code into said four-bit binary code.

1I. A system for magnetically recording and reproducing digital data comprising:

lll

8 a recording medium on ttl'riclt o first bint/ry dici! my lie represented os a change in t/ie polarity of sont nn'illrnn and a second binary digit may be represented as llie absence of n cltongc in tln' polari/y of snit! merlin/n;

on encoder for converting a four-bit binary code to a i'e-bt't binary code allowing combination of two .sito cessive bits only as said second binary digit;

recording means connected to said encoder for recording digital data in said five-bit binary code on sain' recording medium.'

playback means for sensing said reversals of polarity of said recording medium and generating pulses in rcsponse thereto;

an oscillator connected to said playback means effective to generate a wave haring a frequency determined by said pulses; and

logic means connected to said playback means and to said oscillator for recovering said digital data.

l2. A system for magnetically recording and reproducing digital data comprising.'

a recording medium;

recording means for recording digital dota in a five-bit binary code on said recording medium, wher-env the state of said recording medium is changed to record only one of a pair of binary digits;

playback means for sensing said changes of state of said recording medium and generating pulses in response thereto;

an oscillator connected to said playback means eective to generate a wave having a frequency determined by said pulses.' and logic means connected to said playback means and to said oscillator for recovering said digital data.

I3. A system for magnetically recording and reproducing digital data comprising:

a recording medium on which a first binary digit be rep resented as a change in a magnetic state of said medium and a second binary digit may be represented as the absence of a change in said magnetic siate of said medium;

an encoder for converting a four-bit binary code to a )ive-bit binary code allowing conzbinationof two snccessive bits only as said second binary digit;

recording means connected to said encoder for recording digital data in said )ive-bit binary code on said recording medium;

playback means for sensing said changes in magnetic state of said recording medium and generating pulses in response thereto;

an oscillator connected to said playback means effective to generate a wave having a frequency determined by said pulses.' and logic means connected to said playback means and to said oscillator for recovering said digital data.

References Cited The following references, cited by the Examiner, are of record in the patented le of this patent or the original patent.

UNITED STATES PATENTS 3,226,685 12/1965 Potter et al. S60-48 3,274,611 9/1966 Brown et al 360-48 3.357,003 12/1967 MacArthur 360-48 3,500,385 3/1970 Padalino et al. 360-48 VINCENT P. CANNEY, Primary Examiner U.S. Cl. X.R. 360-48. 

